Simultaneous multi-site recordings and iontophoretic drug and dye applications along the trigeminal system of anesthetized rats

Non-NMDA receptor-mediated vibrotactile responses of neurons from the hindpaw representation in the rat SI cortex

Non-NMDA receptor-mediated vibrotactile responses of neurons from the hindpaw representation were investigated in the rat SI cortex. We recorded single-unit spikes evoked by sinusoidal (duration: 500 ms; frequency: 5, 40, and 250 Hz; amplitude: 100 μm peak-to-peak) stimulation of the glabrous skin. The responses were obtained with microinjection of aCSF (sham), bicuculline, and AMPA near the isolated neurons in anaesthetized rats. Blocking most of the NMDA receptors by ketamine revealed local dynamics differentially modulated by each drug. The responses were generally suppressed after the initial 100-ms period of the 40- and 250-Hz stimulus, but not at 5 Hz. Both drugs increased average firing rates (AFRs) only during vibrotactile stimulation, and increased entrainment as measured by the vector strength (VS) of spike phases. However, bicuculline was more effective on the AFR in the late period particularly at 40 Hz. Complex interactions were found with AMPA; late activity increased only for fast spiking neurons at 40 Hz, and more for regular spiking neurons at 5 Hz. The increase of VS by bicuculline was much higher in layer IV. In addition to thalamocortical feed-forward inhibition, vibrotactile information seems to be suppressed after 100 ms by longer-latency inhibitory networks tuned to mid-frequency inputs. Combined with the presumed AMPA-receptor desensitization, those two inhibitory factors could limit the excitatory flow mostly to lower frequencies. The frequency dependence of the drug effects highlights the role of local cortical dynamics in the hindpaw area.

Coupling multielectrode array recordings with silver labeling of recording sites to study cervical spinal network connectivity

Midcervical spinal interneurons form a complex and diffuse network and may be involved in modulating phrenic motor output. The intent of the current work was to enable a better understanding of midcervical "network-level" connectivity by pairing the neurophysiological multielectrode array (MEA) data with histological verification of the recording locations. We first developed a method to deliver 100-nA currents to electroplate silver onto and subsequently deposit silver from electrode tips after obtaining midcervical (C3-C5) recordings using an MEA in anesthetized and ventilated adult rats. Spinal tissue was then fixed, harvested, and histologically processed to "develop" the deposited silver. Histological studies verified that the silver deposition method discretely labeled (50-μm resolution) spinal recording locations between laminae IV and X in cervical segments C3-C5. Using correlative techniques, we next tested the hypothesis that midcervical neuronal discharge patterns are temporally linked. Cross-correlation histograms produced few positive peaks (5.3%) in the range of 0-0.4 ms, but 21.4% of neuronal pairs had correlogram peaks with a lag of ≥0.6 ms. These results are consistent with synchronous discharge involving mono- and polysynaptic connections among midcervical neurons. We conclude that there is a high degree of synaptic connectivity in the midcervical spinal cord and that the silver-labeling method can reliably mark metal electrode recording sites and "map" interneuron populations, thereby providing a low-cost and effective tool for use in MEA experiments. We suggest that this method will be useful for further exploration of midcervical network connectivity.NEW & NOTEWORTHY We describe a method that reliably identifies the locations of multielectrode array (MEA) recording sites while preserving the surrounding tissue for immunohistochemistry. To our knowledge, this is the first cost-effective method to identify the anatomic locations of neuronal ensembles recorded with a MEA during acute preparations without the requirement of specialized array electrodes. In addition, evaluation of activity recorded from silver-labeled sites revealed a previously unappreciated degree of connectivity between midcervical interneurons.

In vivo minimally invasive interstitial multi-functional microendoscopy

Developing minimally invasive methodologies for imaging of internal organs is an emerging field in the biomedical examination research. This paper introduces a new multi-functional microendoscope device capable of imaging of internal organs with a minimal invasive intervention. In addition, the developed microendoscope can also be employed as a monitoring device for measuring local hemoglobin concentration in blood stream when administrated into a blood artery. The microendoscope device has a total external diameter of only 200 μm and can provide high imaging resolution capability of more than 5,000 pixels. The device can detect features with a spatial resolution of less than 1 μm. The microendoscope has been tested both in-vitro as well as in-vivo in rats presenting a promising and powerful tool as a high resolution and minimally invasive imaging facility suitable for previously unreachable clinical modalities.

An inexpensive drivable cannulated microelectrode array for simultaneous unit recording and drug infusion in the same brain nucleus of behaving rats

Neurons are functionally segregated into discrete populations that perform specific computations. These computations, mediated by neuron-neuron electrochemical signaling, form the neural basis of behavior. Thus fundamental to a brain-based understanding of behavior is the precise determination of the contribution made by specific neurotransmitters to behaviorally relevant neural activity. To facilitate this understanding, we have developed a cannulated microelectrode array for use in behaving rats that enables simultaneous neural ensemble recordings and local infusion of drugs in the same brain nucleus. The system is inexpensive, easy to use, and produces robust and quantitatively reproducible drug effects on recorded neurons.

Independent positioning of microelectrodes for multisite recordings in vitro

A robust and easy to handle, inexpensive multisite recording system is described which allows independent positioning of several microelectrodes with high precision axial movement of each electrode. The basic units of the system are a manually operated manipulator for left-right and up-down movement and a micrometer for positioning in the direction of the micrometer axis. The micrometer is actuated with a DC-motor operated by a stand-alone controller module allowing for remote control of the motor in either step mode or continuous mode. The multisite recording system has been proven to allow stable simultaneous recordings of single unit and population activities, extracellular ion concentrations and intracellular potentials in organotypic hippocampal slice cultures (OHSCs) of rat.

Lemniscal and Extralemniscal Compartments in the VPM of the Rat

The ventral posteromedial thalamic nucleus (VPM) of the rat contains at least two major vibrissa-representing compartments: the dorsomedial (VPMdm), which belongs to the lemniscal afferent pathway, and the ventrolateral (VPMvl), which belongs to the extralemniscal afferent pathway. Although input–output projections and functional characteristics that distinguish these two compartments were recently clarified, a comprehensive structural analysis of these compartments and the border between them was lacking. This paper addresses structural and functional relationships between the VPMdm and VPMvl. We found that the size of the VPM is almost constant across individual rats. Next, we computed a canonical map of the VPM in the oblique plane, where structural borders are best visualized. Using the canonical map, and sequential slices cut in oblique and coronal planes, we determined the border between the VPMdm and VPMvl in the standard coronal plane, and verified it with in vivo extracellular recordings. The position of the border between these two vibrissal sub-nuclei changes along the rostrocaudal extent within the VPM due to the relative sizes of these sub-nuclei at any point. The border between the VPMdm and VPMvl, which was revealed by this technique, can now be included in atlases of the rat brain and should facilitate experimental correlation of tactile functions with thalamic regions.

Neural Signature of Taste Familiarity in the Gustatory Cortex of the Freely Behaving Rat

Ample data indicate that the gustatory cortex (GC) subserves the processing, encoding, and storage of taste information. To further elucidate the neural processes involved, we recorded multi-unit activity in the GC of the freely behaving rat as it became familiar with a novel tastant. Exposure to the tastant was performed over three 40- to 50-min sessions, 24 h apart. In each session, the tastant was presented repeatedly, 1 s at a time, with 10- to 12-s inter-trial intervals. The neural response to the tastant typically lasted 7 s. Our results show that the average neuronal response to the tastant increased as this tastant became familiar, but this increase was detected only during the last 5 s of the response. The increased response was not generalized to another tastant. Furthermore, our analysis suggests that specific neuronal populations subserve the processing of familiarity of specific tastants. The signature of familiarity was not detected in the course of the familiarization session, but only on the subsequent day, suggesting that its development involves slow post-acquisition processes. Our data are in line with the notion that GC neurons process multiple taste attributes, familiarity included, during different temporal phases of their response. The data also suggest that by default the brain considers a taste stimulus as novel, unless proven otherwise.

Encoding of vibrissal active touch

Mammals acquire much of their sensory information by actively moving their sensory organs. Yet, the principles of encoding by active sensing are not known. Here we investigated the encoding principles of active touch by rat whiskers (vibrissae). We induced artificial whisking in anesthetized rats and recorded from first-order neurons in the trigeminal ganglion. During active touch, first-order trigeminal neurons presented a rich repertoire of responses, which could not be inferred from their responses to passive deflection stimuli. Individual neurons encoded four specific events: whisking, contact with object, pressure against object, and detachment from object. Whisking-responsive neurons fired at specific deflection angles, reporting the actual whiskers' position with high precision. Touch-responsive neurons encoded the horizontal coordinate of objects' position by spike timing. These findings suggest two specific encoding-decoding schemes for horizontal object position in the vibrissal system.

Imaging spatiotemporal dynamics of surround inhibition in the barrels somatosensory cortex

Sensory processing and its perception require that local information would also be available globally. Indeed, in the mammalian neocortex, local excitation spreads over large distances via the long-range horizontal connections in layer 2/3 and may spread over an entire cortical area if excitatory polysynaptic pathways are also activated. Therefore, a balance between local excitation and surround inhibition is required. Here we explore the spatiotemporal aspects of cortical depolarization and hyperpolarization of rats anesthetized with urethane. New voltage-sensitive dyes (VSDs) were used for high-resolution real-time visualization of the cortical responses to whisker deflections and cutaneous stimulations of the whisker pad. These advances facilitated imaging of ongoing activity and evoked responses even without signal averaging. We found that the motion of a single whisker evoked a cortical response exhibiting either one or three phases. During a triphasic response, there was first a cortical depolarization in a small cortical region the size of a single cortical barrel. Subsequently, this depolarization increased and spread laterally in an oval manner, preferentially along rows of the barrel field. During the second phase, the amplitude of the evoked response declined rapidly, presumably because of recurrent inhibition. Subsequently, the third phase exhibiting a depolarization rebound was observed and clear, and approximately 16 Hz oscillations were detected. Stimulus conditions revealing a net surround hyperpolarization during the second phase were also found. By using new, improved VSD, the present findings shed new light on the spatial parameters of the intricate spatiotemporal cortical interplay of inhibition and excitation.

Figuring space by time

Sensory information is encoded both in space and in time. Spatial encoding is based on the identity of activated receptors, while temporal encoding is based on the timing of activation. In order to generate accurate internal representations of the external world, the brain must decode both types of encoded information, even when processing stationary stimuli. We review here evidence in support of a parallel processing scheme for spatially and temporally encoded information in the tactile system and discuss the advantages and limitations of sensory-derived temporal coding in both the tactile and visual systems. Based on a large body of data, we propose a dynamic theory for vision, which avoids the impediments of previous dynamic theories.

Temporal frequency of whisker movement. I. Representations in brain stem and thalamus

How does processing of information change the internal representations used in subsequent stages of sensory pathways? To approach this question, we studied the representations of whisker movements in the lemniscal and paralemniscal pathways of the rat vibrissal system. We recently suggested that these two pathways encode movement frequency in different ways. We proposed that paralemniscal thalamocortical circuits, functioning as phase-locked loops (PLLs), translate temporally coded information into a rate code. Here we focus on the two major trigeminal nuclei of the brain stem, nucleus principalis and subnucleus interpolaris, and on their thalamic targets, the ventral posteromedial nucleus (VPM) and the medial division of the posterior nucleus (POm). This is the first study in which these brain stem and thalamic nuclei were explored together in the same animals and using the same stimuli. We studied both single- and multi-unit activity. We moved the whiskers both mechanically and by air puffs; here we present air-puff-induced movements because they are more similar to natural movements than movements induced by mechanical stimulations. We describe the basic properties of the responses in these brain stem and thalamic nuclei. The responses in both brain stem nuclei were similar; responses to air puffs were mostly tonic and followed the trajectory of whisker movement. The responses in the two thalamic nuclei were similar during low-frequency stimulations or during the first pulses of high-frequency stimulations, exhibiting more phasic responses than those of brain stem neurons. However, with frequencies >2 Hz, VPM and POm responses differed, generating different representations of the stimulus frequency. In the VPM, response amplitudes (instantaneous firing rates) and spike counts (total number of spikes per stimulus cycle) decreased as a function of the frequency. In the POm, latencies increased and spike count decreased as a function of the frequency. Having described the basic response properties in the four nuclei, we then focus on a specific test of our PLL hypothesis for coding in the paralemniscal pathway. We used short-duration air puffs, much shorter than whisker movements during natural whisking. The activity in this situation was consistent with the prediction we made on the basis of the PLL hypothesis.

Size gradients of barreloids in the rat thalamus

The spatial organization of the anatomical structures along the trigeminal afferent pathway of the rat conserves the topographical order of the receptor sheath: The brainstem barrelettes, thalamic barreloids, and cortical barrels all reflect the arrangement of whiskers across the mystacial pad. Although both the amount of innervation in the mystacial pad and the size of cortical barrels were shown previously to exhibit increasing gradients toward the ventral and caudal whiskers, whether similar gradients existed in the brainstem and thalamus was not known. Here, the authors investigated the size gradients of the barreloids in the ventral posteromedial nucleus of the rat thalamus. Because the angles used to cut the brain were crucial to this study, the optimal cutting angles were determined first for visualization of individual barreloids and of the entire barreloid field. Individual barreloids, arcs, and rows as well as entire barreloid fields were clearly visualized using cytochrome oxidase staining of brain slices that were cut with the optimal cutting angles. For the first five arcs (including straddlers), the length of barreloids increased in the direction of dorsal-to-ventral whiskers and of caudal-to-rostral whiskers. These gradients reveal an inverse relationship between the size of barreloids and whiskers (length and follicle diameter) along arcs and rows. The largest barreloids in the ventral posteromedial nucleus were those that represent whiskers C2-C4, D2-D4, and E2-E4, which are neither the largest nor the most innervated whiskers in the mystacial pad. This implies that the extended representation is not merely a reflection of peripheral innervation biases and probably serves an as yet unknown processing function.

Transformation from temporal to rate coding in a somatosensory thalamocortical pathway

The anatomical connections from the whiskers to the rodent somatosensory (barrel) cortex form two parallel (lemniscal and paralemniscal) pathways. It is unclear whether the paralemniscal pathway is directly involved in tactile processing, because paralemniscal neuronal responses show poor spatial resolution, labile latencies and strong dependence on cortical feedback. Here we show that the paralemniscal system can transform temporally encoded vibrissal information into a rate code. We recorded the representations of the frequency of whisker movement along the two pathways in anaesthetized rats. In response to varying stimulus frequencies, the lemniscal neurons exhibited amplitude modulations and constant latencies. In contrast, paralemniscal neurons in both thalamus and cortex coded the input frequency as changes in latency. Because the onset latencies increased and the offset latencies remained constant, the latency increments were translated into a rate code: increasing onset latencies led to lower spike counts. A thalamocortical loop that includes cortical oscillations and thalamic gating can account for these results. Thus, variable latencies and effective cortical feedback in the paralemniscal system can serve the processing of temporal sensory cues, such as those that encode object location during whisking. In contrast, fixed time locking in the lemniscal system is crucial for reliable spatial processing.